Is this a rebranding of GDAX? I am hoping that the existing GDAX APIs will remain functional, because some of my apps and scripts are relying on them.

As far as I can see (if you click on API it will redirect to GDAX) it's just GDAX rebranded and officalized as Coinbase Market for limit orders.

Will my GDAX account still work after this?

Pretty much use Coinbase Pro (formerly GDAX) for most of my virtual currency trading. Really enjoy the low platform fees and high trading volumes as it means I get more bang for my crypto-buck!

I hope this will make filing taxes easier too!

I stopped using GDAX a while back after I got screwed a few too many times by their server load problems, regardless, it's still missing a hand full of TA tools.